While working on my valved radio set, I used a tool which I first encountered at the tender age of 18 on the roof of the electrical eng building at Imperial College. The ham radio soc (HAMSOC) were having a go at a contest in the autumn, the younger hams had put up a nest of dipoles (40-80-160m) which would not tune up.
An older ham (the late Jimmy Bolton G3HBN) appeared and he set the young Mark Foreman to work, in those days I was not licensed. I was waiting for a chance to sit the license exam. I was sent to hunt for insulators on the roof. The people who had put up the dipoles had tied the ends of the wires directly to metal parts on the roof.
I found some nylon rope and some other bits of plastic, we then added these makeshift insulators to the aerial. Then jimmy got out a small box and pulled out a GDO and a small coil he fitted to the feed point of the dipoles. He quickly found the resonant frequency of the dipoles with this simple tool, at that point I realised that I wanted a GDO.
I made up my mind to buy one from Maplin, but before I could save up and go and buy one they stopped selling GDOs. Then years later I tried to build my own one, this was a big waste of time as the two transistor circuit I used could not support an osscilation over a wide range of frequencies.
Then I bought a MFJ unit back in 2007, this has proven to be a very useful tool. When building wire aerials I tend to use it to help me prune the aerials using a real antenna tuning unit (wirecutters). Note that a transmatch and a SPC match are examples of antenna matching units.
The other great use of a GDO is as a signal source and with the oscillator turned off as a adsorption wavemeter. It was as a signal source that I last used by GDO.